Method and apparatus for video coding

ABSTRACT

Aspects of the disclosure provide methods, apparatuses, and non-transitory computer-readable storage mediums for video encoding/decoding. An apparatus includes processing circuitry that decodes prediction information for a current block in a coded video sequence. The prediction information indicates a cross-component linear model (CCLM) prediction mode for the current block. The processing circuitry determines a sample value of a first unavailable neighboring luma sample of the current block based on at least one luma sample used in the CCLM prediction mode not being available. The sample value of the first unavailable neighboring luma sample is determined based on a sample value of an available neighboring luma sample. The processing circuitry calculates a parameter of the CCLM prediction mode based on the sample value of the first unavailable neighboring luma sample of the current block. Further, the processing circuitry reconstructs the current block based on the calculated parameter of the CCLM prediction mode.

INCORPORATION BY REFERENCE

This present application claims the benefit of priority to U.S.Provisional Application No. 63/006,552, “SIMPLIFICATION ON CROSSCOMPONENT LINEAR MODEL PREDICTION,” filed on Apr. 7, 2020, and U.S.Provisional Application No. 63/011,901, “SIMPLIFICATION ON CROSSCOMPONENT LINEAR MODEL PREDICTION,” filed on Apr. 17, 2020, which areincorporated by reference herein in their entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate) of, for example, 60 picturesper second or 60 Hz. Uncompressed video has significant bitraterequirements. For example, 1080p604:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless and lossy compression, as well as a combination thereof can beemployed. Lossless compression refers to techniques where an exact copyof the original signal can be reconstructed from the compressed originalsignal. When using lossy compression, the reconstructed signal may notbe identical to the original signal, but the distortion between originaland reconstructed signals is small enough to make the reconstructedsignal useful for the intended application. In the case of video, lossycompression is widely employed. The amount of distortion tolerateddepends on the application; for example, users of certain consumerstreaming applications may tolerate higher distortion than users oftelevision distribution applications. The compression ratio achievablecan reflect that: higher allowable/tolerable distortion can yield highercompression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is only using reference data from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or maybe predicted itself.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (105) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar MV derived from MVs of a neighboringarea. That results in the MV found for a given area to be similar or thesame as the MV predicted from the surrounding MVs, and that in turn canbe represented, after entropy coding, in a smaller number of bits thanwhat would be used if coding the MV directly. In some cases, MVprediction can be an example of lossless compression of a signal(namely: the MVs) derived from the original signal (namely: the samplestream). In other cases, MV prediction itself can be lossy, for examplebecause of rounding errors when calculating a predictor from severalsurrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described herein is atechnique henceforth referred to as “spatial merge.”

Referring to FIG. 1C, a current block (111) can include samples thathave been found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (112 through 116, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide apparatuses for videoencoding/decoding. An apparatus includes processing circuitry thatdecodes prediction information for a current block in a current picturethat is a part of a coded video sequence. The prediction informationindicates a cross-component linear model (CCLM) prediction mode for thecurrent block. The processing circuitry determines a sample value of afirst unavailable neighboring luma sample of the current block based onat least one luma sample used in the CCLM prediction mode not beingavailable. The sample value of the first unavailable neighboring lumasample is determined based on a sample value of an available neighboringluma sample. The processing circuitry calculates a parameter of the CCLMprediction mode based on the sample value of the first unavailableneighboring luma sample of the current block. Further, the processingcircuitry reconstructs the current block based on the calculatedparameter of the CCLM prediction mode.

In an embodiment, the available neighboring luma sample is located in arow above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample and in a rowadjacent to the row in which the available neighboring luma sample islocated.

In an embodiment, the available neighboring luma sample is located in arow above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample in the samerow and top-left of the current block.

In an embodiment, the available neighboring luma sample is located in acolumn left of the current block and the first unavailable neighboringluma sample is adjacent to the available neighboring luma sample in thesame column and top-left of the current block.

In an embodiment, the processing circuitry determines a sample value ofa second unavailable neighboring luma sample of the current block basedon the sample value of the available neighboring luma sample. Theprocessing circuitry calculates the parameter of the CCLM predictionmode based on the sample value of the first unavailable neighboring lumasample and the sample value of the second unavailable neighboring lumasample of the current block.

In an embodiment, the available neighboring luma sample, the firstunavailable neighboring luma sample, and the second unavailableneighboring luma sample are located in a same column and different rowsabove the current block.

In an embodiment, the processing circuitry performs a down-sample filteron the sample value of the first unavailable neighboring luma sample ofthe current block. The processing circuitry calculates the parameter ofthe CCLM prediction mode based on a result of the down-sample filter.

In an embodiment, the first unavailable neighboring luma sample of thecurrent block is located above the current block, and the processingcircuitry performs an N-tap filter on the first unavailable neighboringluma sample. N is determined based on whether the first unavailableneighboring luma sample and a corresponding chroma sample are verticalcollocated.

Aspects of the disclosure provide methods for video encoding/decoding.In the method, prediction information is decoded for a current block ina current picture that is a part of a coded video sequence. Theprediction information indicates a CCLM prediction mode for the currentblock. A sample value of a first unavailable neighboring luma sample ofthe current block is determined based on at least one luma sample usedin the CCLM prediction mode not being available The sample value of thefirst unavailable neighboring luma sample is determined based on asample value of an available neighboring luma sample. A parameter of theCCLM prediction mode is calculated based on the sample value of thefirst unavailable neighboring luma sample of the current block. Thecurrent block is reconstructed based on the calculated parameter of theCCLM prediction mode.

Aspects of the disclosure also provide non-transitory computer-readablemediums storing instructions which when executed by a computer for videodecoding cause the computer to perform any one or a combination of themethods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes;

FIG. 1B is an illustration of exemplary intra prediction directions;

FIG. 1C is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example;

FIG. 2 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system in accordance with an embodiment;

FIG. 4 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment;

FIG. 5 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment;

FIG. 6 shows a block diagram of an encoder in accordance with anotherembodiment;

FIG. 7 shows a block diagram of a decoder in accordance with anotherembodiment;

FIG. 8 shows exemplary locations of neighboring chroma samples andcorresponding neighboring luma samples used for a derivation of across-component linear model (CCLM) parameters;

FIGS. 9A and 9B show exemplary down-sample filtering processes for leftand top neighboring luma samples, respectively;

FIGS. 10A and 10B show two examples of padding top neighboring lumasamples for the derivation of the CCLM parameters;

FIGS. 11A and 11B show two examples of padding top-left neighboring lumasamples for the derivation of the CCLM parameters;

FIGS. 12A and 12B show two examples of padding top-left neighboring lumasamples for the derivation of the CCLM parameters;

FIG. 13 shows an exemplary flowchart in accordance with an embodiment;and

FIG. 14 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

I. Video Decoder and Encoder Systems

FIG. 2 illustrates a simplified block diagram of a communication system(200) according to an embodiment of the present disclosure. Thecommunication system (200) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (250). Forexample, the communication system (200) includes a first pair ofterminal devices (210) and (220) interconnected via the network (250).In the FIG. 2 example, the first pair of terminal devices (210) and(220) performs unidirectional transmission of data. For example, theterminal device (210) may code video data (e.g., a stream of videopictures that are captured by the terminal device (210)) fortransmission to the other terminal device (220) via the network (250).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (220) may receive the codedvideo data from the network (250), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (200) includes a secondpair of terminal devices (230) and (240) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (230) and (240)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (230) and (240) via the network (250). Eachterminal device of the terminal devices (230) and (240) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (230) and (240), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 2 example, the terminal devices (210), (220), (230) and(240) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (250) represents any number ofnetworks that convey coded video data among the terminal devices (210),(220), (230) and (240), including for example wireline (wired) and/orwireless communication networks. The communication network (250) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(250) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 3 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick, and the like.

A streaming system may include a capture subsystem (313) that caninclude a video source (301), for example a digital camera, creating forexample a stream of video pictures (302) that are uncompressed. In anexample, the stream of video pictures (302) includes samples that aretaken by the digital camera. The stream of video pictures (302),depicted as a bold line to emphasize a high data volume when compared toencoded video data (304) (or coded video bitstreams), can be processedby an electronic device (320) that includes a video encoder (303)coupled to the video source (301). The video encoder (303) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (304) (or encoded video bitstream (304)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (302), can be stored on a streamingserver (305) for future use. One or more streaming client subsystems,such as client subsystems (306) and (308) in FIG. 3 can access thestreaming server (305) to retrieve copies (307) and (309) of the encodedvideo data (304). A client subsystem (306) can include a video decoder(310), for example, in an electronic device (330). The video decoder(310) decodes the incoming copy (307) of the encoded video data andcreates an outgoing stream of video pictures (311) that can be renderedon a display (312) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (304),(307), and (309) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (320) and (330) can includeother components (not shown). For example, the electronic device (320)can include a video decoder (not shown) and the electronic device (330)can include a video encoder (not shown) as well.

FIG. 4 shows a block diagram of a video decoder (410) according to anembodiment of the present disclosure. The video decoder (410) can beincluded in an electronic device (430). The electronic device (430) caninclude a receiver (431) (e.g., receiving circuitry). The video decoder(410) can be used in the place of the video decoder (310) in the FIG. 3example.

The receiver (431) may receive one or more coded video sequences to bedecoded by the video decoder (410); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (401), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (431) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (431) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (415) may be coupled inbetween the receiver (431) and an entropy decoder/parser (420) (“parser(420)” henceforth). In certain applications, the buffer memory (415) ispart of the video decoder (410). In others, it can be outside of thevideo decoder (410) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (410), forexample to combat network jitter, and in addition another buffer memory(415) inside the video decoder (410), for example to handle playouttiming. When the receiver (431) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (415) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (415) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (410).

The video decoder (410) may include the parser (420) to reconstructsymbols (421) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (410),and potentially information to control a rendering device such as arender device (412) (e.g., a display screen) that is not an integralpart of the electronic device (430) but can be coupled to the electronicdevice (430), as was shown in FIG. 4. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (420) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (420) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (420) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, MVs, and so forth.

The parser (420) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (415), so as tocreate symbols (421).

Reconstruction of the symbols (421) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (420). The flow of such subgroup control information between theparser (420) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (410)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (451). Thescaler/inverse transform unit (451) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (421) from the parser (420). The scaler/inversetransform unit (451) can output blocks comprising sample values that canbe input into aggregator (455).

In some cases, the output samples of the scaler/inverse transform (451)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (452). In some cases, the intra pictureprediction unit (452) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (458). The currentpicture buffer (458) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(455), in some cases, adds, on a per sample basis, the predictioninformation that the intra prediction unit (452) has generated to theoutput sample information as provided by the scaler/inverse transformunit (451).

In other cases, the output samples of the scaler/inverse transform unit(451) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (453) canaccess reference picture memory (457) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (421) pertaining to the block, these samples can beadded by the aggregator (455) to the output of the scaler/inversetransform unit (451) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (457) from where themotion compensation prediction unit (453) fetches prediction samples canbe controlled by MVs, available to the motion compensation predictionunit (453) in the form of symbols (421) that can have, for example X, Y,and reference picture components. Motion compensation also can includeinterpolation of sample values as fetched from the reference picturememory (457) when sub-sample exact MVs are in use, MV predictionmechanisms, and so forth.

The output samples of the aggregator (455) can be subject to variousloop filtering techniques in the loop filter unit (456). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (456) as symbols (421) from the parser (420), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (456) can be a sample stream that canbe output to the render device (412) as well as stored in the referencepicture memory (457) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (420)), the current picture buffer (458) can becomea part of the reference picture memory (457), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (410) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (431) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (410) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 5 shows a block diagram of a video encoder (503) according to anembodiment of the present disclosure. The video encoder (503) isincluded in an electronic device (520). The electronic device (520)includes a transmitter (540) (e.g., transmitting circuitry). The videoencoder (503) can be used in the place of the video encoder (303) in theFIG. 3 example.

The video encoder (503) may receive video samples from a video source(501) (that is not part of the electronic device (520) in the FIG. 5example) that may capture video image(s) to be coded by the videoencoder (503). In another example, the video source (501) is a part ofthe electronic device (520).

The video source (501) may provide the source video sequence to be codedby the video encoder (503) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any colorspace (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (501) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (501) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (503) may code andcompress the pictures of the source video sequence into a coded videosequence (543) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (550). In some embodiments, the controller(550) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (550) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum MV allowed reference area, and soforth. The controller (550) can be configured to have other suitablefunctions that pertain to the video encoder (503) optimized for acertain system design.

In some embodiments, the video encoder (503) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (530) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (533)embedded in the video encoder (503). The decoder (533) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (534). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (534) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (533) can be the same as of a“remote” decoder, such as the video decoder (410), which has alreadybeen described in detail above in conjunction with FIG. 4. Brieflyreferring also to FIG. 4, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (545) and the parser (420) can be lossless, the entropy decodingparts of the video decoder (410), including the buffer memory (415) andthe parser (420) may not be fully implemented in the local decoder(533).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (530) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously-coded picture fromthe video sequence that were designated as “reference pictures”. In thismanner, the coding engine (532) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (533) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (530). Operations of the coding engine (532) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 5), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (533) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (534). In this manner, the video encoder(503) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (535) may perform prediction searches for the codingengine (532). That is, for a new picture to be coded, the predictor(535) may search the reference picture memory (534) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture MVs, block shapes, and so on, that may serve as an appropriateprediction reference for the new pictures. The predictor (535) mayoperate on a sample block-by-pixel block basis to find appropriateprediction references. In some cases, as determined by search resultsobtained by the predictor (535), an input picture may have predictionreferences drawn from multiple reference pictures stored in thereference picture memory (534).

The controller (550) may manage coding operations of the source coder(530), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (545). The entropy coder (545)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (540) may buffer the coded video sequence(s) as createdby the entropy coder (545) to prepare for transmission via acommunication channel (560), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(540) may merge coded video data from the video coder (503) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (550) may manage operation of the video encoder (503).During coding, the controller (550) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most one MVand reference index to predict the sample values of each block.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two MVs and reference indices to predict the sample values of eachblock. Similarly, multiple-predictive pictures can use more than tworeference pictures and associated metadata for the reconstruction of asingle block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (503) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (503) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (540) may transmit additional datawith the encoded video. The source coder (530) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a MV. The MV points to thereference block in the reference picture, and can have a third dimensionidentifying the reference picture, in case multiple reference picturesare in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first MV that points to a first reference block in the firstreference picture, and a second MV that points to a second referenceblock in the second reference picture. The block can be predicted by acombination of the first reference block and the second reference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quad-tree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 6 shows a diagram of a video encoder (603) according to anotherembodiment of the disclosure. The video encoder (603) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (603) is used in theplace of the video encoder (303) in the FIG. 3 example.

In an HEVC example, the video encoder (603) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (603) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (603) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(603) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the MV is derived from one or more MVpredictors without the benefit of a coded MV component outside thepredictors. In certain other video coding technologies, a MV componentapplicable to the subject block may be present. In an example, the videoencoder (603) includes other components, such as a mode decision module(not shown) to determine the mode of the processing blocks.

In the FIG. 6 example, the video encoder (603) includes the interencoder (630), an intra encoder (622), a residue calculator (623), aswitch (626), a residue encoder (624), a general controller (621), andan entropy encoder (625) coupled together as shown in FIG. 6.

The inter encoder (630) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, MVs, merge mode information), and calculate inter predictionresults (e.g., prediction block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (622) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (622) also calculates intra prediction results (e.g., predictionblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (621) is configured to determine general controldata and control other components of the video encoder (603) based onthe general control data. In an example, the general controller (621)determines the mode of the block, and provides a control signal to theswitch (626) based on the mode. For example, when the mode is the intramode, the general controller (621) controls the switch (626) to selectthe intra mode result for use by the residue calculator (623), andcontrols the entropy encoder (625) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(621) controls the switch (626) to select the inter prediction resultfor use by the residue calculator (623), and controls the entropyencoder (625) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (623) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (622) or the inter encoder (630). Theresidue encoder (624) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (624) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (603) also includes a residuedecoder (628). The residue decoder (628) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (622) and theinter encoder (630). For example, the inter encoder (630) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (622) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (625) is configured to format the bitstream toinclude the encoded block. The entropy encoder (625) is configured toinclude various information according to a suitable standard such asHEVC. In an example, the entropy encoder (625) is configured to includethe general control data, the selected prediction information (e.g.,intra prediction information or inter prediction information), theresidue information, and other suitable information in the bitstream.Note that, according to the disclosed subject matter, when coding ablock in the merge submode of either inter mode or bi-prediction mode,there is no residue information.

FIG. 7 shows a diagram of a video decoder (710) according to anotherembodiment of the disclosure. The video decoder (710) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (710) is used in the place of the videodecoder (310) in the FIG. 3 example.

In the FIG. 7 example, the video decoder (710) includes an entropydecoder (771), an inter decoder (780), a residue decoder (773), areconstruction module (774), and an intra decoder (772) coupled togetheras shown in FIG. 7.

The entropy decoder (771) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (772) or the inter decoder (780), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (780); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (772). The residual information can be subject to inversequantization and is provided to the residue decoder (773).

The inter decoder (780) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (772) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (773) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (773) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (771) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (774) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (773) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (303), (503), and (603), and thevideo decoders (310), (410), and (710) can be implemented using anysuitable technique. In an embodiment, the video encoders (303), (503),and (603), and the video decoders (310), (410), and (710) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (303), (503), and (603), and the videodecoders (310), (410), and (710) can be implemented using one or moreprocessors that execute software instructions.

II. Cross Component Linear Model Prediction

In some related examples such as VVC, to reduce a cross-componentredundancy, a cross-component linear model (CCLM) prediction mode isused. In the CCLM prediction mode, chroma samples of a CU can bepredicted based on reconstructed luma samples of the same CU by using alinear model as follows:

pred_C(i,j)=α·rec_L′(i,j)+β

where pred_C(i,j) represents the predicted chroma samples in the CU andrec_L(i,j) represents down-sampled reconstructed luma samples of the CU.

FIG. 8 shows exemplary locations of neighboring chroma samples andcorresponding neighboring luma samples used for the derivation of theCCLM parameters (e.g., α and β). In FIG. 8, a luma block (810) isalready reconstructed and a chroma block (800) corresponding to the lumablock is being predicted by using the CCLM prediction mode. The CCLMparameters are derived based the neighboring chroma samples of thechroma block (800) and the corresponding neighboring luma samples of theluma block (810). For example, a neighboring chroma sample (801) and acorresponding neighboring luma sample (811) can be used for thederivation of the CCLM parameters.

In some examples, the CCLM parameters can be derived with at most fourneighboring chroma samples of a CU and corresponding neighboring lumasamples of the CU. FIG. 8 also shows down-sampled luma samplescorresponding to the neighboring chroma samples. The correspondingneighboring luma samples can be used directly or padded for adown-sample filtering process. For the neighboring luma samples locatedin the top (or above), left, and top-left areas of the CU, whenever theneighboring luma samples are reconstructed and available, they can beused in the down-sample filtering process.

FIGS. 9A and 9B show exemplary down-sample filtering processes for leftand top neighboring luma samples, respectively. In both FIGS. 9A and 9B,an SPS chroma vertical collocated flag such asSPS_chroma_vertical_collocated_flag is used to indicate whether a chromasample and a corresponding luma sample in a CU are vertical collocated.In some examples, if the SPS chroma vertical collocated flag is true, itindicates that the chroma sample and the corresponding luma sample inthe CU are vertical collocated. If the SPS chroma vertical collocatedflag is false, it indicates that the chroma sample and the correspondingluma sample in the CU are not vertical collocated. For example, for apicture with a chroma sampling pattern of 4:4:4, the SPS chroma verticalcollocated flag can be true.

In FIG. 9A, for the left neighboring luma samples of the CU, if the SPSchroma vertical collocated flag is true, a 5-tap down-sample filter canbe used for the left neighboring luma samples. If the SPS chromavertical collocated flag is false, a 6-tap down-sample filter can beused for the left neighboring luma samples.

In FIG. 9B, for the top (or above) neighboring luma samples of the CU,the down-sample filter can be selected based on the SPS chroma verticalcollocated flag and a location of the CU. If the CU is adjacent to aboundary of a CTU in which the CU is located, a 3-tap down-sample filtercan be used for the top neighboring luma samples. If the CU is notadjacent to the CTU boundary, the down-sample filter can be determinedbased on whether the SPS chroma vertical collocated flag is true. If theSPS chroma vertical collocated flag is true, the 5-tap down-samplefilter can be used for the top neighboring luma samples. If the SPSchroma vertical collocated flag is false, the 6-tap down-sample filtercan be used for the top neighboring luma samples.

III. Improvements on Cross Component Linear Model Prediction

As described above, in some cases such as for the top neighboring lumasamples, the down-sample filter process depends on the location of theCU, leading to a more complicated process. This disclosure includesimprovements to the CCLM prediction mode.

According to aspects of the disclosure, when a neighboring luma sampleof a CU used in a derivation of the CCLM parameters is unavailable, asample value of the unavailable neighboring luma sample can bedetermined based on a sample value of an available neighboring lumasample of the CU. For example, the unavailable neighboring luma samplecan be padded using the available neighboring luma sample.

FIGS. 10A and 10B show two examples of padding top neighboring lumasamples for the derivation of the CCLM parameters. The top neighboringluma samples include available and unavailable neighboring luma samples.The unavailable top neighboring luma samples can be padded using theavailable neighboring luma samples.

In some embodiments, when M rows of top neighboring luma samples areneeded for the derivation of the CCLM parameters, and N (which is lessthan M) rows of top neighboring luma samples are available, M-N row(s)is(are) padded using one of the N rows, for example the row that isclosest to the M rows of top neighboring luma samples. In an embodiment,M can be predetermined.

In FIG. 10A, M and N are equal to 4 and 1, respectively. Thus, for acurrent luma block (1000), top rows Line 0-Line 3 are needed for thederivation of the CCLM parameters. Line 0 is a row of available topneighboring luma samples and Line 1-Line 3 are three rows of unavailabletop neighboring luma samples. Line 1-Line 3 can be padded using Line 0row by row. For example, three unavailable top neighboring luma samples(1002)-(1004) in Line 1-Line 3 can be padded using an available topneighboring luma sample (1001) in Line 0. Thus, all the threeunavailable top neighboring luma samples (1002)-(1004) can have the samesample value of K, which is a sample value of the available topneighboring luma sample (1001).

In FIG. 10B, M and N are equal to 4 and 2, respectively. Thus, for acurrent luma block (1010), top rows Line 0-Line 3 are needed for thederivation of the CCLM parameters. Line 0-Line 1 are two rows ofavailable top neighboring luma samples and Line 2-Line 3 are two rows ofunavailable top neighboring luma samples. Line 2-Line 3 can be paddedusing Line 1 row by row. For example, two unavailable top neighboringluma samples (1013)-(1014) in Line 2-Line 3 can be padded using anavailable top neighboring luma sample (1012) in Line 1, respectively.Thus, both unavailable top neighboring luma samples (1013)-(1014) canhave the same sample value of A1, which is a sample value of theavailable top neighboring luma sample (1012).

As described above, by padding unavailable neighboring luma samplesusing available neighboring luma samples, the down-sample filter processfor top neighboring luma samples can be independent of a CTU boundary.That is, a location of a CU is not needed to be examined in thedown-sample filter process for the top neighboring luma samples. Thus,the down-sample filter process for the top neighboring luma samples canonly depend on the SPS chroma vertical collocated flag for example. Inan embodiment, if the SPS chroma vertical collocated flag is true, a5-tap filter such as the existing 5 tap filter in FIG. 9B can be usedfor the derivation of the CCLM parameters. If the SPS chroma verticalcollocated flag is false, a 6-tap filter such as the existing 6 tapfilter in FIG. 9B can be used for the derivation of the CCLM parameters.

According to aspects of the disclosure, if a reconstructed top-leftneighboring luma sample of a CU is used in the derivation of the CCLMparameters, instead of using a reconstructed sample value of thetop-left neighboring luma sample, the sample value of the top-leftneighboring luma sample can be determined based on a sample value of atop neighboring luma sample or a sample value of a left neighboring lumasample of the CU. For example, the top-left neighboring luma sample canbe padded using the top neighboring luma sample or the left neighboringluma sample. Using the padded sample value instead of the reconstructedsample value for the top-left neighboring luma sample can be beneficialfor a hardware implementation since the reconstructed top-leftneighboring luma sample may be stored in a register separated from thereconstructed top or left neighboring luma samples in some embodiments.

FIGS. 11A and 11B show two examples of padding top-left neighboring lumasamples for the derivation of the CCLM parameters. In both FIGS. 11A and11B, the top-left neighboring luma samples can be reconstructed whenderiving the CCLM parameters.

In an embodiment, the reconstructed top-left neighboring luma samplesare not used in the down-sample filter process of the top neighboringluma samples when deriving the CCLM parameters. For the down-samplefilter process of the top neighboring luma samples, the availability ofthe left and top-left neighboring luma samples can be marked asunavailable. For example, only reconstructed top neighboring lumasamples can be used in the down-sample filter process of the topneighboring luma samples. Whenever the down-sample filter process of thetop neighboring luma samples requires an unavailable luma sample locatedin N[x, y], where x<0, the unavailable luma sample can be padded withanother neighboring luma sample (e.g., nearest neighboring luma sample).For example, the unavailable luma sample in N[x, y] can be padded with anearest reconstructed top neighboring luma sample M[j, k], where k=y andj>=0.

In FIG. 11A, for a current luma block (1100), an unavailable top-leftneighboring luma sample (1101) located in [−1, −2] can be padded usingan available top neighboring luma sample (1102) located in [0, −2] witha sample value of B0. Thus, a sample value of the unavailable top-leftneighboring luma sample (1101) can be B0. The unavailable top-leftneighboring luma sample (1101) is located adjacent to the available topneighboring luma sample (1102). Both luma samples (1101) and (1102) arein one row above the current luma block (1100).

In an embodiment, the reconstructed top-left neighboring luma samplesare not used in the down-sample filter process of the left neighboringluma samples when deriving the CCLM parameters. For the down-samplefilter process of the left neighboring luma samples, the availability ofthe top and top-left neighboring luma samples can be marked asunavailable. For example, only reconstructed left neighboring lumasamples can be used in the down-sample filter process of the leftneighboring luma samples. Whenever the down-sample filter process of theleft neighboring luma samples requires an unavailable luma sample inlocated in O[x, y], where y<0, the unavailable luma sample can be paddedwith another neighboring luma sample (e.g., nearest neighboring lumasample). For example, the unavailable luma sample in O[x, y] can bepadded with a nearest reconstructed left neighboring luma sample P[j,k], where j=x and k>=0.

In FIG. 11B, for a current luma block (1110), an unavailable top-leftneighboring luma sample (1111) located in [−2, −1] can be padded usingan available left neighboring luma sample (1112) located in [−2, 0] witha sample value of B0. The unavailable top-left neighboring luma sample(1111) is located adjacent to the available left neighboring luma sample(1112). Both luma samples (1111) and (1112) are in one column left tothe current block (1110).

In an embodiment, the reconstructed top-left neighboring luma samplesare not used in the down-sample filter process of the top-leftneighboring luma samples when deriving the CCLM parameters. For thedown-sample filter process of the top-left neighboring luma samples, theavailability of the top-left neighboring luma samples can be marked asunavailable. For example, one of the reconstructed top and leftneighboring luma samples can be used in the down-sample filter processof the top-left neighboring luma samples. Use of the top or leftneighboring luma samples can be based on the down-sample filter process.In an example, if the down-sample filter process is first performed onthe top neighboring luma samples and then followed by the leftneighboring luma samples, the top-left neighboring luma samples can bedetermined based on the top neighboring luma samples. In anotherexample, if the down-sample filter process is first performed on theleft neighboring luma samples and then followed by the top neighboringluma samples, the top-left neighboring luma samples can be determinedbased on the left neighboring luma samples.

FIGS. 12A and 12B show two examples of padding top-left neighboring lumasamples for the derivation of the CCLM parameters. In both FIGS. 12A and12B, the top-left neighboring luma samples can be reconstructed butmarked as unavailable when deriving the CCLM parameters. The down-samplefilter process can require 4 rows of top neighboring luma samples. Inboth FIGS. 12A and 12B, 2 rows of top neighboring luma samples areavailable and the other 2 rows of top neighboring luma samples areunavailable. The unavailable rows can be padded using a nearestavailable row of top neighboring luma samples.

In FIG. 12A, the down-sample filter process can be first performed onthe top neighboring luma samples and then followed by the leftneighboring luma samples. For example, for a current luma block (1200),in the down-sample filter process of the top neighboring luma samples,unavailable top-left neighboring luma samples (1201) and (1202) can bepadded with available top neighboring luma samples (1206) and (1205),respectively. Both unavailable top neighboring luma samples (1203) and(1204) can be padded with the available top neighboring luma sample(1205). For example, the neighboring luma sample (1201) has a samplevalue of D and each of the neighboring luma samples (1202)-(1204) has asample value of D1.

In FIG. 12B, the down-sample filter process can be first performed onthe left neighboring luma samples and then followed by the topneighboring luma samples. For example, for a current luma block (1210),in the down-sample filter process of the left neighboring luma samples,unavailable top-left neighboring luma samples (1213) and (1214) can bepadded with available left neighboring luma samples (1212) and (1211),respectively. Then, in the down-sample filter process of the topneighboring luma samples, both unavailable top neighboring luma samples(1215) and (1216) can be padded with an available top neighboring lumasample (1217). Thus, the neighboring luma sample (1213) has a samplevalue of B0, the neighboring luma sample (1214) has a sample value of B,and each of the neighboring luma samples (1215) and (1216) has a samplevalue of D1.

IV. Flowchart

FIG. 13 shows a flow chart outlining an exemplary process (1300)according to an embodiment of the disclosure. In various embodiments,the process (1300) is executed by processing circuitry, such as theprocessing circuitry in the terminal devices (210), (220), (230) and(240), the processing circuitry that performs functions of the videoencoder (303), the processing circuitry that performs functions of thevideo decoder (310), the processing circuitry that performs functions ofthe video decoder (410), the processing circuitry that performsfunctions of the intra prediction module (452), the processing circuitrythat performs functions of the video encoder (503), the processingcircuitry that performs functions of the predictor (535), the processingcircuitry that performs functions of the intra encoder (622), theprocessing circuitry that performs functions of the intra decoder (772),and the like. In some embodiments, the process (1300) is implemented insoftware instructions, thus when the processing circuitry executes thesoftware instructions, the processing circuitry performs the process(1300).

The process (1300) may generally start at step (S1310), where theprocess (1300) decodes prediction information for a current block in acurrent picture that is a part of a coded video sequence. The predictioninformation indicates a CCLM prediction mode for the current block.Then, the process (1300) proceeds to step (S1320).

At step (S1320), the process (1300) determines a sample value of a firstunavailable neighboring luma sample of the current block based on atleast one luma sample used in the CCLM prediction mode not beingavailable. The sample value of the first unavailable neighboring lumasample can be determined based on a sample value of an availableneighboring luma sample. Then, the process (1300) proceeds to step(S1330).

At step (S1330), the process (1300) calculates a parameter of the CCLMprediction mode based on the sample value of the first unavailableneighboring luma sample of the current block. Then, the process (1300)proceeds to step (S1340).

At step (S1340), the process (1300) reconstructs the current block basedon the calculated parameter of the CCLM prediction mode. Then, theprocess (1300) terminates.

In an embodiment, the available neighboring luma sample is located in arow above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample and in a rowadjacent to the row in which the available neighboring luma sample islocated.

In an embodiment, the available neighboring luma sample is located in arow above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample in the samerow and top-left of the current block.

In an embodiment, the available neighboring luma sample is located in acolumn left of the current block and the first unavailable neighboringluma sample is adjacent to the available neighboring luma sample in thesame column and top-left of the current block.

In an embodiment, the processing circuitry determines a sample value ofa second unavailable neighboring luma sample of the current block basedon the sample value of the available neighboring luma sample. Theprocessing circuitry calculates the parameter of the CCLM predictionmode based on the sample value of the first unavailable neighboring lumasample and the sample value of the second unavailable neighboring lumasample of the current block.

In an embodiment, the available neighboring luma sample, the firstunavailable neighboring luma sample, and the second unavailableneighboring luma sample are located in a same column and different rowsabove the current block.

In an embodiment, the processing circuitry performs a down-sample filteron the sample value of the first unavailable neighboring luma sample ofthe current block. The processing circuitry calculates the parameter ofthe CCLM prediction mode based on a result of the down-sample filter.

In an embodiment, the first unavailable neighboring luma sample of thecurrent block is located above the current block, and the processingcircuitry performs an N-tap filter on the first unavailable neighboringluma sample. N is determined based on whether the first unavailableneighboring luma sample and a corresponding chroma sample are verticalcollocated.

V. Computer System

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 14 shows a computersystem (1400) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 14 for computer system (1400) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1400).

Computer system (1400) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1401), mouse (1402), trackpad (1403), touchscreen (1410), data-glove (not shown), joystick (1405), microphone(1406), scanner (1407), camera (1408).

Computer system (1400) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1410), data-glove (not shown), or joystick (1405), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1409), headphones(not depicted)), visual output devices (such as screens (1410) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability—some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted). These visual output devices (such as screens(1410)) can be connected to a system bus (1448) through a graphicsadapter (1450).

Computer system (1400) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1420) with CD/DVD or the like media (1421), thumb-drive (1422),removable hard drive or solid state drive (1423), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1400) can also include a network interface (1454) toone or more communication networks (1455). The one or more communicationnetworks (1455) can for example be wireless, wireline, optical. The oneor more communication networks (1455) can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of the one or more communication networks (1455) includelocal area networks such as Ethernet, wireless LANs, cellular networksto include GSM, 3G, 4G, 5G, LTE and the like, TV wireline or wirelesswide area digital networks to include cable TV, satellite TV, andterrestrial broadcast TV, vehicular and industrial to include CANBus,and so forth. Certain networks commonly require external networkinterface adapters that attached to certain general purpose data portsor peripheral buses (1449) (such as, for example USB ports of thecomputer system (1400)); others are commonly integrated into the core ofthe computer system (1400) by attachment to a system bus as describedbelow (for example Ethernet interface into a PC computer system orcellular network interface into a smartphone computer system). Using anyof these networks, computer system (1400) can communicate with otherentities. Such communication can be uni-directional, receive only (forexample, broadcast TV), uni-directional send-only (for example CANbus tocertain CANbus devices), or bi-directional, for example to othercomputer systems using local or wide area digital networks. Certainprotocols and protocol stacks can be used on each of those networks andnetwork interfaces as described above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1440) of thecomputer system (1400).

The core (1440) can include one or more Central Processing Units (CPU)(1441), Graphics Processing Units (GPU) (1442), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1443), hardware accelerators for certain tasks (1444), and so forth.These devices, along with Read-only memory (ROM) (1445), Random-accessmemory (1446), internal mass storage (1447) such as internal non-useraccessible hard drives, SSDs, and the like, may be connected through thesystem bus (1448). In some computer systems, the system bus (1448) canbe accessible in the form of one or more physical plugs to enableextensions by additional CPUs, GPU, and the like. The peripheral devicescan be attached either directly to the core's system bus (1448), orthrough a peripheral bus (1449). Architectures for a peripheral businclude PCI, USB, and the like.

CPUs (1441), GPUs (1442), FPGAs (1443), and accelerators (1444) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1445) or RAM (1446). Transitional data can be also be stored in RAM(1446), whereas permanent data can be stored for example, in theinternal mass storage (1447). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1441), GPU (1442), massstorage (1447), ROM (1445), RAM (1446), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1400), and specifically the core (1440) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1440) that are of non-transitorynature, such as core-internal mass storage (1447) or ROM (1445). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1440). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1440) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1446) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1444)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

Appendix A: Acronyms AMVP: Advanced Motion Vector Prediction ASIC:Application-Specific Integrated Circuit ATMVP: Alternative/AdvancedTemporal Motion Vector Prediction BMS: Benchmark Set BV: Block VectorCANBus: Controller Area Network Bus CB: Coding Block CD: Compact DiscCPR: Current Picture Referencing CPUs: Central Processing Units CRT:Cathode Ray Tube CTBs: Coding Tree Blocks CTUs: Coding Tree Units CU:Coding Unit DPB: Decoder Picture Buffer DVD: Digital Video Disc FPGA:Field Programmable Gate Areas GOPs: Groups of Pictures GPUs: GraphicsProcessing Units GSM: Global System for Mobile communications HEVC: HighEfficiency Video Coding HRD: Hypothetical Reference Decoder IBC: IntraBlock Copy IC: Integrated Circuit JEM: Joint Exploration Model LAN:Local Area Network LCD: Liquid-Crystal Display LTE: Long-Term EvolutionMV: Motion Vector OLED: Organic Light-Emitting Diode PBs: PredictionBlocks PCI: Peripheral Component Interconnect PLD: Programmable LogicDevice PUs: Prediction Units RAM: Random Access Memory ROM: Read-OnlyMemory SCC: Screen Content Coding SEI: Supplementary EnhancementInformation SNR: Signal Noise Ratio SSD: Solid-state Drive TUs:Transform Units USB: Universal Serial Bus VUI: Video UsabilityInformation VVC: Versatile Video Coding

What is claimed is:
 1. A method of video decoding in a decoder,comprising: decoding prediction information for a current block in acurrent picture that is a part of a coded video sequence, the predictioninformation indicating a cross-component linear model (CCLM) predictionmode for the current block; determining a sample value of a firstunavailable neighboring luma sample of the current block based on atleast one luma sample used in the CCLM prediction mode not beingavailable, the sample value of the first unavailable neighboring lumasample being determined based on a sample value of an availableneighboring luma sample; calculating a parameter of the CCLM predictionmode based on the sample value of the first unavailable neighboring lumasample of the current block; and reconstructing the current block basedon the calculated parameter of the CCLM prediction mode.
 2. The methodof claim 1, wherein the available neighboring luma sample is located ina row above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample and in a rowadjacent to the row in which the available neighboring luma sample islocated.
 3. The method of claim 1, wherein the available neighboringluma sample is located in a row above the current block and the firstunavailable neighboring luma sample is adjacent to the availableneighboring luma sample in the same row and top-left of the currentblock.
 4. The method of claim 1, wherein the available neighboring lumasample is located in a column left of the current block and the firstunavailable neighboring luma sample is adjacent to the availableneighboring luma sample in the same column and top-left of the currentblock.
 5. The method of claim 1, wherein the determining includesdetermining a sample value of a second unavailable neighboring lumasample of the current block based on the sample value of the availableneighboring luma sample, and the calculating includes calculating theparameter of the CCLM prediction mode based on the sample value of thefirst unavailable neighboring luma sample and the sample value of thesecond unavailable neighboring luma sample of the current block.
 6. Themethod of claim 5, wherein the available neighboring luma sample, thefirst unavailable neighboring luma sample, and the second unavailableneighboring luma sample are located in a same column and different rowsabove the current block.
 7. The method of claim 1, wherein thecalculating includes: performing a down-sample filter on the samplevalue of the first unavailable neighboring luma sample of the currentblock; and calculating the parameter of the CCLM prediction mode basedon a result of the down-sample filter.
 8. The method of claim 7, whereinthe first unavailable neighboring luma sample of the current block islocated above the current block, and the performing the down-samplefilter includes performing an N-tap filter on the first unavailableneighboring luma sample, N being determined based on whether the firstunavailable neighboring luma sample and a corresponding chroma sampleare vertical collocated.
 9. An apparatus, comprising processingcircuitry configured to: decode prediction information for a currentblock in a current picture that is a part of a coded video sequence, theprediction information indicating a cross-component linear model (CCLM)prediction mode for the current block; determine a sample value of afirst unavailable neighboring luma sample of the current block based onat least one luma sample used in the CCLM prediction mode not beingavailable, the sample value of the first unavailable neighboring lumasample being determined based on a sample value of an availableneighboring luma sample; calculate a parameter of the CCLM predictionmode based on the sample value of the first unavailable neighboring lumasample of the current block; and reconstruct the current block based onthe calculated parameter of the CCLM prediction mode.
 10. The apparatusof claim 9, wherein the available neighboring luma sample is located ina row above the current block and the first unavailable neighboring lumasample is adjacent to the available neighboring luma sample and in a rowadjacent to the row in which the available neighboring luma sample islocated.
 11. The apparatus of claim 9, wherein the available neighboringluma sample is located in a row above the current block and the firstunavailable neighboring luma sample is adjacent to the availableneighboring luma sample in the same row and top-left of the currentblock.
 12. The apparatus of claim 9, wherein the available neighboringluma sample is located in a column left of the current block and thefirst unavailable neighboring luma sample is adjacent to the availableneighboring luma sample in the same column and top-left of the currentblock.
 13. The apparatus of claim 9, wherein the processing circuitry isconfigured to: determine a sample value of a second unavailableneighboring luma sample of the current block based on the sample valueof the available neighboring luma sample, and calculate the parameter ofthe CCLM prediction mode based on the sample value of the firstunavailable neighboring luma sample and the sample value of the secondunavailable neighboring luma sample of the current block.
 14. Theapparatus of claim 13, wherein the available neighboring luma sample,the first unavailable neighboring luma sample, and the secondunavailable neighboring luma sample are located in a same column anddifferent rows above the current block.
 15. The apparatus of claim 9,wherein the processing circuitry is configured to: perform a down-samplefilter on the sample value of the first unavailable neighboring lumasample of the current block; and calculate the parameter of the CCLMprediction mode based on a result of the down-sample filter.
 16. Theapparatus of claim 15, wherein the first unavailable neighboring lumasample of the current block is located above the current block, and theprocessing circuitry is configured to: perform an N-tap filter on thefirst unavailable neighboring luma sample, N being determined based onwhether the first unavailable neighboring luma sample and acorresponding chroma sample are vertical collocated.
 17. Anon-transitory computer-readable storage medium storing instructionsexecutable by at least one processor to perform: decoding predictioninformation for a current block in a current picture that is a part of acoded video sequence, the prediction information indicating across-component linear model (CCLM) prediction mode for the currentblock; determining a sample value of a first unavailable neighboringluma sample of the current block based on at least one luma sample usedin the CCLM prediction mode not being available, the sample value of thefirst unavailable neighboring luma sample being determined based on asample value of an available neighboring luma sample; calculating aparameter of the CCLM prediction mode based on the sample value of thefirst unavailable neighboring luma sample of the current block; andreconstructing the current block based on the calculated parameter ofthe CCLM prediction mode.
 18. The non-transitory computer-readablestorage medium of claim 17, wherein the available neighboring lumasample is located in a row above the current block and the firstunavailable neighboring luma sample is adjacent to the availableneighboring luma sample and in a row adjacent to the row in which theavailable neighboring luma sample is located.
 19. The non-transitorycomputer-readable storage medium of claim 17, wherein the availableneighboring luma sample is located in a row above the current block andthe first unavailable neighboring luma sample is adjacent to theavailable neighboring luma sample in the same row and top-left of thecurrent block.
 20. The non-transitory computer-readable storage mediumof claim 17, wherein the available neighboring luma sample is located ina column left of the current block and the first unavailable neighboringluma sample is adjacent to the available neighboring luma sample in thesame column and top-left of the current block.